1. Field of the Invention
The present invention relates to polymeric films used for a wide variety of purposes. More particularly, the present invention is related to polymeric films that act as substrates or facestock for labels, and many other film-based products. Still more particularly, the present invention is related to monolayer polymeric film products which are subsequently printed with a variety of printing systems.
2. Description of the Prior Art
It is well known that labels, tapes, decals, etc., are widely-accepted means for conveying information. For many years, the information was printed on a thin paper product that either had a pressure-sensitive adhesive backing, or that was otherwise adhere to the surface of interest. Paper was used, and continues to be used, because it has sufficient flexibility and toughness to conform to various surfaces without tearing-at least without tearing immediately. A further advantage of paper is its ability to accept print applied in the variety of ways that print is applied, including, but not limited to, offset, thermal transfer, letterpress, rotogravure, flex, and screen. That is, the print can easily be applied without concern that it will separate from the paper substrate.
At least one disadvantage in the use of paper as a label substrate is its moisture absorbency. This characteristic can cause a reduction in tear strength as well as a reduction in shelf life. Other problems with paper labels include curling of the material and the general aesthetics of the paper. In order to overcome these and other disadvantages in the use of paper as a label substrate, viscoelastic, or polymeric, films have been developed. Polymeric films--including those made with polyethylene or polypropylene as the foundation component--have been developed as substitutes for paper label substrates. This material is relatively inexpensive to obtain and to fabricate in the thickness range of interest and noted. It also has a higher tear strength than paper. Further, it is much more moisture resistant that paper. Two disadvantages of note, however, include the ability of polymeric films to conform to flexible items to which they are affixed, and the difficulty in getting polymeric films to accept print of the types described.
Fairly recent developments in polymeric film fabrication have addressed the problem of conformance to flexible surfaces. That problem occurs, for example, when a label is affixed to a bottle or container made of a flexible material--such as a plastic beverage container. The label must be sufficiently flexible to conform to changes in the container surface, when, for example, the bottle is squeezed. The label must, however, be tough enough to remain in tact under those conditions so that the information thereon remains visible. The primary means for addressing this issue has been the introduction of a film-processing step that substantially aligns the molecular structure of the polymer as it is being stretched to its desired thickness.
It is important to understand in considering the processing of these types of films that the material used is a thermoplastic polymer. That is, the material can be heated to a temperature high enough to cause it to become fluid-like without being a liquid, so that it may be made into virtually any desired shape and is much more "handleable" than cast thermoset polymers. The shaping of the material is achieved by mechanically stretching or orienting the polymer chains in a particular way. As the material is chilled, it solidifies into the desired shape, albeit with some shrinkage as crosslinking occurs. Thermoplastic materials may be formed and re-formed in this way into a variety of shapes. Thermoset polymers, on the other hand, can only be processed once. Any subsequent attempts to heat them for reforming result in irreversible degradation.
In the field of film substrates, the processing of the thermoplastic polymer can be arranged such that the desired film shape-principally an issue of thickness--is achieved, along with the conformability and strength needed for the flexible containers noted. The shaping and strengthening is achieved by stretching an extruded film that is initially about 10-12 mils thick over a plurality of rollers so as to thin it while also orienting the polymeric chains substantially in a single direction. This processing technique has been available for quite some time. One example of it is described in U.S. Pat. No. 5,186,782 issued to Freedman. The Freedman patent describes the orientation of a facestock film so as to provide greater stiffness in that direction while maintaining conformability.
The problem with the prior-art means for producing a conformable film involves the printability of the film. Inherent in the nature of the polymeric materials used to produce these films is the lack of a suitable film surface profile to which print can adhere. In general, fabricated polyethylene and polypropylene lacks sufficient "nooks and crannies" to which print ink can adhere. As a result, the surface of the material must be treated, or a more suitable material must be applied to the surface of the polymer. In the field of plastic films, labels, etc., treatment of the polymer surface is either too expensive, or it simply cannot be done for the film thickness range indicated. Therefore, the industry standard at present is to co-extrude at least a second layer of material with the primary facestock layer, where the second layer more readily accepts print, but does not have the strength of the primary material. Further, it is fairly common for that second material to be applied to both sides of the primary material. The label stock for printing described in the Freedman patent describes this layering of different materials.
It has been determined that vinyl-acetate, generally in an ethylene-vinylacetate (EVA) complex, will accept print of the type commonly used throughout the printing field--whether oil- or water-based. By forming a multi-layered film, it is possible to produce a relatively inexpensive, printable, and conformable substrate that is of interest to the label industry.
A significant drawback to this multi-layer processing, however, relates to the quality of the film provided. Specifically, the layering of two unlike materials, each of which has its own set of optimal processing conditions, yields an inferior product. In particular, the primary material, which is typically polypropylene, but may be polyethylene or a co-polymer of polypropylene and polyethylene, can and should be heated to temperatures on the order of 270.degree. F. 295.degree. F. for proper orientation processing. Processing at lower temperatures reduces stabilization of the crystalline polymer and thereby reduces the resultant stiffness, flatness, and curl of the film. EVA, on the other hand, must not be processed at a temperature greater than 250.degree. F. when it is formed as its own distinctive layer. Further, the two dissimilar materials orient (flow) and shrink differently as they are heated, cooled, and solidify. In fact, the amorphous EVA shrinks in a different way than the crystalline polypropylene, particularly as it approaches the polypropylene processing temperature of 250.degree. F. Because of this dissimilarity, delamination and curling of the multi-layer film is common. In addition, processing at lower temperatures creates a tension in the structural matrix of the film that can only be relieved by natural curling of the film. As a result of these two problems, a label or other type of identifier having such a film facestock is extremely difficult to print, and causes considerable waste as the film either separates, curls, or a combination of the two. Unfortunately, this problem cannot be avoided in that the layering of materials for print labeling forces the manufacturer to process at temperatures no greater than 250.degree. F.
Therefore, what is needed is a polymeric film substrate that is conformable to an array of surfaces, flexible or otherwise. What is also needed is a polymeric film substrate for labels, tapes, etc., to which a variety of printing inks will adhere. Further, what is needed is a polymeric film substrate that will not delaminate and which is substantially curl-free. Yet further, what is needed is a polymeric film substrate that can be processed under optimal conditions and with little, if any, increase in the cost typically associated with such fabrication.